This disclosure relates to lithography using pulsed laser illumination In particular it relates to lithography for producing electronic devices on wafers using multi-mode excimer and molecular lasers, e.g. KrF, ArF, and F2 lasers. It may also apply to illumination systems where several single-mode sources are mixed or one single-mode laser beam is split and recombined with time delays, thereby creating an equivalent multimode source. EUV sources do not give much speckle, but similar illumination micro-nonuniformities may be created by the illuminator optics and the invention may have utility also for EUV lithography.
The inventor and co-workers have shown that the coherent character of multi-mode laser light causes significant micro-nonuniformity (“speckle”), both static and dynamic, in the mask plane of an exposure system. The dynamic part comes from the light source and the static part may be created by the optics. Knowledge about actual levels of speckle and the understanding of their causes in real lithographic systems is still incomplete. The amount of non-uniformity affects the CD uniformity of the image and ultimately the yield of the production process. (Reference 1: Christer Rydberg, Jörgen Bengtsson and Tor Sandström, “Dynamic Laser Speckle as a Detrimental Phenomenon in Optical Projection Lithography,” Journal of Microlithography, Microfabrication, and Microsystems, Vol 5, 033004-1-8, (2006)). Other sources of micro-nonuniformities in the mask plane are interference or incomplete mixing in the illuminator. These nonuniformities may have similar lateral spectra as the speckle described in Reference 1, but may be more difficult to average out by multiple pulses.
The level of speckle, and, generally, micro-nonuniformities may be influenced by design and operating conditions. The micrononuniformity can be made better by sacrificing the through-put, increasing the exposure dose, modifying the coherence, and reducing the degree of polarization. However, in order to make these trade-offs, a way to measure the actual amount of micrononuniformity is needed. Currently no such method exists. Makers of exposure equipment may be doing lab experiments, but there is no way for the users of the equipment to quantify the micro-nonuniformities in process illumination. In prior art, exposure systems have detectors for verifying the illumination uniformity, but these detectors typically measure the average light energy over many laser pulses, multiple scanning positions, and across a finite area. Above all, they do not have the lateral resolution needed to quantify the speckle-like micrononuniformities. In a system for printing 50 nm lines width there may be micro-nonuniformities with sizes below 100 nm in the wafer plane. Therefore, current detectors for illumination uniformity do not give a true image of the local micro-nonuniformity which has a spatial spectrum nearly as high as the printed pattern. It is the purpose of the current disclosure to describe methods and apparatuses for detecting and quantifying micro-nonuniformities in lithography, and make such measurements possible in a production setting with extreme requirements of cleanliness and little tolerance of side effects from the measurement.